The need for color matchability of coated articles (before heat treatment vs. after heat treatment) is known. Glass substrates are often produced in large quantities and cut to size in order to fulfill the needs of a particular situation such as a new multi-window and door office building, vehicle window needs, etc. It is often desirable in such applications that some of the windows and/or doors be heat treated (i.e., tempered, heat strengthened or bent), while others need not be. Office buildings often employ IG units and/or laminates for safety and/or thermal control. It is often desirable that the units and/or laminates which are heat treated (HT) substantially match their non-heat treated counterparts (e.g., with regard to color, reflectance, and/or the like) for architectural and/or aesthetic purposes.
U.S. Pat. No. 5,376,455 discloses a coated article including: glass/Si3N4/NiCr/Ag/NiCr/Si3N4. Unfortunately, the coating system of the '455 patent is not sufficiently color matchable after heat treatment with its non-heat-treated counterpart. In other words, the coating system of the '455 patent has a rather high ΔE value. This means that, unfortunately, two different coated articles with different coatings (one to be heat treated, the other not to be) must be made for customers who want their heat-treated and non-heat-treated coated articles to approximately match colorwise as viewed by the naked eye.
As with the '455 patent, it has mostly been possible to achieve matchability only by providing two different layer systems, one of which is heat treated (HT) and the other is not. The necessity of developing and using two different layer systems to achieve matchability creates additional manufacturing expense and inventory needs which are undesirable.
However, commonly owned U.S. Pat. No. 5,688,585 discloses a solar control coated article including glass/Si3N4/NiCr/Si3N4, wherein matchability is achieved with a single layer system. As explained at column 9 of the '585 patent, it is a “requirement” of the '585 invention that the NiCr layer be substantially free of any nitride. An object of the '585 patent is to provide a sputter coated layer system that after heat treatment is matchable colorwise with its non-heat-treated counterpart. However, the '585 patent uses a heat treatment (HT) of only three (3) minutes (col. 10, line 55). Longer heat treatments are often desired in order to attain better tempering or HT characteristics. Unfortunately, as explained below, it has been found that with longer HT times the coatings of the '585 patent cannot maintain low ΔE values and thus lose color matchability. In particular, it has surprisingly been found by the instant inventor that in coatings such as that of the '585 patent, ΔE values jump significantly upward after HT for 4-5 minutes at a temperature of from about 600 to 800 degrees C.
Consider the following layer stack (see Example 7 below): glass/Si3N4/NiCr/Si3N4, where the underlayer of Si3N4 is about 50-70 Å (angstroms) thick, the NiCr layer is about 325 Å thick (the NiCr layer is not nitrided as deposited as can be seen in FIG. 15), and the overcoat of Si3N4 is about 210-310 Å thick. As explained in Example 7 below, this coated article has a rather high transmissive ΔE* value of about 5.9 after a heat treatment (HT) at 625 degrees C. for ten (10) minutes. This high transmissive ΔE value means that a HT version of the '585 coated article does not approximately match colorwise non-heat-treated counterpart versions with regard to transmissive color after 10 minutes of HT. This is not desirable.
The instant inventor believes that the high ΔE* value associated with the coating of Example 7 herein is caused for at least the following reasons. FIG. 15 is an XPS plot illustrating the Example 7 coating before heat treatment (HT), while FIG. 16 illustrates the Example 7 coating after HT. As shown in FIG. 15, before heat treatment the three different layers are fairly separate and distinct. For example, prior to HT it can be seen that the Ni slopes 3 on either side of the NiCr layer are very steep, as are the Si and N slopes 5 and 7, respectively, on the lower side of the upper Si3N4 layer. Therefore, the vast majority of the Ni is located in the NiCr layer and the vast majority of the Si and N from the upper Si3N4 layer is located in that layer. However, FIG. 16 illustrates that when the FIG. 15 coated article of Example 7 is heat treated (HT) for 10 minutes as discussed above, a significant portion of the Ni from the NiCr layer migrates into the upper Si3N4 layer. Additionally, upon HT a significant portion of the Si and N from the upper Si3N4 layer migrates into the NiCr layer. In other words, the interface between the metal NiCr layer and the upper Si3N4 layer becomes blurred and non-distinct. This is evidenced in FIG. 16 by the less steep slope 3a of the Ni on the upper/outer side of the NiCr layer, and by the less steep slopes 5a and 7a of the Si and N on the lower side of the upper Si3N4 layer. Still further, it can be seen by comparing FIGS. 15 and 16 that HT causes a significant amount of the Cr in the NiCr layer to migrate within that layer toward the upper side thereof so that it is not as uniformly distributed compared to pre-HT.
Unfortunately, the aforesaid migrations of the Si, N, Ni and Cr from their FIG. 15 positions to their respective FIG. 16 positions due to HT causes significant color shifting to occur and thus explains the large transmissive ΔE* value associated with the coating of Ex. 7, and thus with coatings of the '585 patent when exposed to lengthy heat treatments.
In view of the above, it will be apparent to those skilled in the art that there exists a need for a coating or layer system that has a low ΔE (or ΔE*) value(s) (transmissive and/or glass side reflective) and thus good color matchability characteristics after at least five (5) minutes of heat treatment (HT). It is a purpose of this invention to fulfill the above-listed need, and/or other needs which will become more apparent to the skilled artisan once given the following disclosure.